1. Field of the Invention
This invention relates to dimensionally stable fibers of a copolyamide consisting essentially of recurring units of the formulas ##STR1## wherein from 50 to 80 mole percent of the units are Formula (I) units.
2. Description of the Prior Art
Polyhexamethylene adipamide (i.e., nylon 66) fibers and polyethylene terephthalate (i.e., PET) fibers are widely used commercially in tire and textile applications. Nylon 66 fibers are considerably more durable than PET fibers and therefore are preferred in applications such as carpets, ropes and other applications where durability is of major concern. In tire applications, both nylon and PET fibers are used in large quantities. Nylon 66 fibers have a lower modulus than PET fibers but are more durable and have better adhesion to rubber characteristics. Tires reinforced with nylon 66, however, are not used as original equipment on automobiles due to their tendency to flatspot. In apparel applications where wrinkle resistance is a major factor, PET fibers are used much more extensively than nylon 66 fibers, even though nylon 66 fibers have superior dye performance. Nylon 66 fibers are easily dyed with acid dyes to bright and brilliant colors, whereas PET fibers are not. The dyeing of PET fibers requires the use of pressure and carriers which adds an extra 8 to 12 cents per pound to the cost of the dyed fibers. Additionally, dyed PET fibers do not possess good light fastness and the use of carriers tends to cause air pollution.
It is apparent from the foregoing discussion that fibers having the best properties of both nylon 66 and PET fibers without the shortcomings of either would be highly desirable and represent a major contribution to the art. Such fibers would have the durability, adhesion to rubber, and dye performance of nylon 66 fibers and the modulus, wrinkle resistance and non-flatspotting characteristics of PET fibers. To have major significance, of course, such fibers should be prepared from available raw materials and should cost no more than either nylon 66 or PET fibers.
A fiber meeting the above-mentioned criteria will be dimensionally stable, that is, it will be molecularly oriented (i.e., drawn) and have crystalline regions as well as amorphous regions in its structure. Essentially amorphous fibers, even though molecularly oriented, lack dimensional stability, and, therefore are not useful. Dimensionally stable fibers have low shrinkage values when subjected to heat, such as when placed in boiling water, and also have good elastic recovery properties. The ability of fibers to develop crystalline regions depends to a great extent on the regularity of their particular structure and the chemical nature of their polymer chains. Since a crystalline region in a polymeric fiber is essentially one where the polymer chains are arranged in an orderly and regular fashion, any factors which decrease the regularity of the polymer chains will also significantly reduce the ability of the fiber to crystallize. Thus, it is not surprising that fibers consisting of a copolymer having two different recurring units will have difficulty in crystallizing, particularly when each of the units makes up at least 10 percent or more of the copolymer. For example, copolyamide fibers consisting of equal portions of nylon 6 and nylon 66 recurring units are essentially amorphous, lack dimensional stability, and cannot be made appreciably more crystalline by techniques such as by drawing and/or heating.
Polyhexamethylene terephthalamide (i.e., 6TA) fibers are crystalline fibers having good properties. However 6TA fibers melt at about 370.degree. C., too high for conventional melt spinning, and hence are prepared by solution spinning techniques using sulfuric acid as the solvent. Recovery of sulfuric acid is expensive and disposal thereof is unacceptable from environmental viewpoints. Polyhexamethylene isophthalamide (i.e., 6IA) fibers are essentially amorphous and thus are not useful. Copolyamide fibers are known to be difficult to crystallize, especially when the comonomers exceed about 10%. Also, shrinkages are generally high with fibers of copolymers. Accordingly, copolyamides in which the comonomers exceed about 10% have not heretofore been used commercially in tire and textile fiber applications, although they are extensively used in coating and molding resin applications.